Synthesis of fluorobenzaldehydes

1. Field of the Invention

The present invention relates to formylation of benzene derivatives so as to form fluorobenzaldehydes.

2. Description of the Background Art

Fluorobenzaldehydes are useful as intermediates for the synthesis of various dyes, herbicidal compounds, pharmaceuticals and the like.

There are several well-known methods for electrophilically formylating arenes which contain activating (electron-donating) substituents, but these either fail completely or are impractical for arenes which contain electron withdrawing substituents such as fluorine. Although little work has been done with alpha, alpha-dichloromethyl alkyl ethers as formylating agents, certain benzene derivatives which contain mildly activating ring substituents have previously been formylated using dichloromethyl alkyl ethers in the presence of tin chloride, titanium chloride, or aluminum chloride. Such reactions have not achieved commercial importance because the isomeric purity of the aldehyde products has been poor. Furthermore, the yields are often reduced by unwanted coupling products between the intermediates and substrate, and commercially useful quantities of dichloromethyl alkyl ethers are not readily available.

There remains a need in the art for methods of synthesizing fluorobenzaldehydes in high isomeric purity, and for new fluorobenzaldehyde compounds.

In accordance with the present invention, a process for synthesizing 4-fluorobenzaldehydes, comprises providing a compound of the formula (I) ##STR3## wherein R 1 , R 2 , R 3 and R 4 can be the same or different, and represent H, halogen, or an alkyl group of from 1 to about 10 carbon atoms, wherein, when at least one of R 1 , R 2 , R 3 or R 4 is halogen, at least one of remaining R 1 , R 2 , R 3 or R 4 is alkyl. A reaction mixture is formed of the compound of formula (I) and a formylating agent in the presence of FeCl 3 . From the reaction mixture is separated a compound of the formula (II) ##STR4## wherein R 1 , R 2 , R 3 and R 4 are as defined above. The invention further relates to compounds of the formula (II) wherein R 1 , R 2 , R 3 and R 4 can be the same or different, and represent H or an alkyl group of from 1 to about 10 carbon atoms, wherein at least 1 of R 1 , R 2 , R 3 or R 4 is alkyl, and wherein when R 1 , R 2 , and R 4 are H, R 3 is an alkyl group of from 2 to about 10 carbon atoms.

A starting material for carrying out the process of the present invention is a compound of the formula (I) as defined above, wherein R 1 , R 2 , R 3 and R 4 can be the same or different, and represent H, a halogen such as fluorine, chlorine, bromine or the like, or an alkyl group of from 1 to about 10 carbon atoms wherein, when at least one of R 1 , R 2 , R 3 or R 4 is halogen, at least one of remaining R 1 , R 2 , R 3 or R 4 is alkyl.

A reaction mixture is formed of the compound of formula (I) as defined above, and a formylating agent in the presence of an FeCl 3 catalyst (which advantageously is anhydrous). From the reaction mixture is separated a 4-fluorobenzaldehyde of the formula (II) as set forth above, wherein R 1 , R 2 , R 3 or R 4 are as previously defined.

In the process of the present invention, the position of the fluorine on the benzene ring in formula (I) directs the positioning of the aldehyde group (--CHO) to the 4-position, so long as substituents R 1 , R 2 , R 3 and R 4 do not interfere.

When R 1 , R 2 , R 3 and/or R 4 are alkyl groups, they can be straight chain or branched. In preferred embodiments, when R 1 , R 2 , R 3 and/or R 4 are alkyl groups, the number of carbon atoms therein are from 1 to about 5, more preferably, from 1 to about 3 and even more preferably about 2 or 3 carbon atoms.

In accordance with one embodiment, one of R 1 , R 2 , R 3 or R 4 is an alkyl group, and the remainder of R 1 , R 2 , R 3 and R 4 are H.

In a preferred embodiment, one of R 2 or R 3 is methyl and meta to F in formula (I), and the remainder of R 1 , R 2 , R 3 or R 4 are H.

In another preferred embodiment, one of R 1 or R 4 is methyl and ortho to F in formula (I), and the remainder of R 1 , R 2 , R 3 and R 4 are H.

In preferred embodiments, the formylating agent with which the compound of formula (I) is reacted, is selected from the group consisting of dichloromethyl alkyl ethers, dichloromethyl alkanoates (e.g., acetates), and dichloromethyl benzoate. In particularly preferred embodiments, the formylating agent is a dichloromethyl alkyl ether, most preferably dichloromethyl methyl ether.

According to one aspect of the invention, a dichloromethyl alkyl ether formylating agent is provided by reacting PCl 5 with an alkyl formate at about room temperature so as to form about an equimolar mixture of the corresponding dichloromethyl alkyl ether and phosphorus oxychloride (POCl 3 ). This mixture is reacted with a compound of formula (I) in the presence of the catalyst FeCl 3 . Hydrolysis of the mixture with water forms the corresponding compound of formula (II). The dichloromethyl alkyl ether preferably is separated from the phosphorous oxychloride by distillation prior to use, but even the equimolar mixture can serve as a formylation agent by the addition of additional FeCl 3 . Any POCl 3 remaining can be separated from the compound of formula (II) by hydrolysis of the POCl 3 .

In one embodiment, the compound of formula (1) is reacted with the formylating agent in the presence of catalyst FeCl 3 . In compounds of formula (I) where formylation results in an isomeric mixture of intermediates, the addition of an electrophilic reagent to the reaction mixture, which already contains catalytic FeCl 3 , results in selective isomeric enrichment. In a preferred embodiment, the reagent is bromine. Despite the elevated temperature typically required, minimal unwanted side products are formed when bromine is used in the presence of FeCl 3 .

In accordance with one embodiment, the reaction of a compound of formula (I) so as to form a compound of the formula (II) takes place in the presence of a halogenated solvent diluent, such as methylene chloride. However, it is not necessary to utilize a halogenated solvent as a diluent, as the formula (I) substrate itself can serve as the diluent.

In accordance with one embodiment, a reaction mixture is formed between the compound of formula (I) and FeCl 3 , to which is added a dichloromethyl alkyl ether at a temperature between about -20° C. and 0° C. Elemental halogen (e.g. bromine) then is added to the reaction mixture, which then is heated to between about 30° C. and about 100° C., preferably to about 65° C. The reaction mixture then is subjected to hydrolysis, after which the desired 4-fluorobenzaldehyde is separated from the reaction mixture, for example by distillation.

Without being bound to any particular theory, it is believed that the FeCl 3 catalyst and the nature of the intermediates in the isomeric enrichment step are primarily responsible for the high yields of substantially isomerically pure 4-fluorobenzaldehydes according to the present invention.

The invention also relates to compounds of the formula (II) set forth above, wherein R 1 , R 2 , R 3 and R 4 can be the same or different, and represent H or an alkyl group of from 1 to about 10 carbon atoms, wherein at least one of R 1 , R 2 , R 3 or R 4 is alkyl, and wherein when R 1 , R 2 , and R 4 are H, R 3 is an alkyl group of from 2 to about 10 carbon atoms. Preferred inventive compounds are those wherein one or more of R 1 , R 2 , R 3 or R 4 is an alkyl group, and the remainder of R 1 , R 2 , R 3 and R 4 are H. A particularly preferred compound in accordance with the present invention is a compound of the formula (II) set forth above wherein R 1 is methyl and ortho to F in formula (II), and R 2 , R 3 and R 4 are H.

The invention is further illustrated by the following examples, which are not intended to be limiting.

About 1.4 equivalents of Dichloromethyl methyl ether was slowly added to fluorobenzene in the presence of approximately one equivalent of anhydrous ferric chloride at -5° C., with methylene chloride as diluent. The dichloromethyl methyl ether was obtained by stirring PCl 5 and methyl formate at room temperature so as to form an equimolar mixture of POCl 3 and dichloromethyl methyl ether. This was added to the fluorobenzene without purification and with additional FeCl 3 .

Unwanted 2-fluorobenzaldehyde isomer was preferentially reacted with very high selectivity and minimal side reactions by introducing approximately 0.4 equivalents of elemental bromine into the reaction mixture and heating to 65° C. for three hours. Hydrolysis of the reaction mixture and distillation provided facile separation of the desired 4-fluorobenzaldehyde from the brominated ortho isomer. The POCl 3 did not interfere with the formylation process, nor did it adversely affect the subsequent isomer enrichment step. Hydrolysis effected the removal of POCl 3 prior to distillation.

The process described in Example I was repeated substantially as described therein, except that no methylene chloride was utilized as diluent, with the fluorobenzene substrate serving as diluent. Excess fluorobenzene was removed prior to the isomeric enrichment step. The results achieved were substantially the same as in Example I.

The process as substantially described in Example II was utilized to convert 2-fluorotoluene to 4-fluoro-3-methyl benzaldehyde, using dichloromethyl methyl ether, ferric chloride and bromine.

Since many modifications, variations, and changes in detail may be made to the described embodiments, it is intended that all matter in the foregoing description be interpreted as illustrative and not in a limiting sense.